It has been proposed, for instance in the U.S. patent application Ser. No. 08/122,615 filed Sep. 16, 1993, now U.S. Pat. No. 5,528,497, and assigned to the same Assignee, to initially define a standard yaw rate response model, compare the actual yaw rate response with a standard yaw rate response computed from the standard yaw rate response model, evaluate external disturbances according to the deviation of the actual yaw rate response from the standard yaw rate response, and produce a steering torque with a powered actuator which tends to cancel the deviation. The contents of this copending application are hereby incorporated in the present application by reference. Thus, when the actual yaw rate is smaller than the standard yaw rate, the steering system automatically produces a steering torque which tends to cancel this deviation. Conversely, when the actual yaw rate is greater than the standard yaw rate, a steering torque is produced which tends to somewhat reduce the steering angle. By doing so, the vehicle is made highly resistant against external disturbances which could arise from crosswind and road surface irregularities. Also, the steering effort required by the vehicle operator is kept at an appropriate level at all times.
However, according to this previous proposal, it is necessary to prepare a standard yaw rate model in advance, and continually compare the actual yaw rate response with a standard yaw rate response. This presents a substantial difficulty in designing the control algorithm, and complicates the structure of the control system.
A yaw rate response of a vehicle is typically expressed by a transfer function involving a second-order delay, and the behavior of the vehicle can be favorably controlled by defining the standard yaw rate response model with such a transfer function. However, when the vehicle operator makes a rapid steering maneuver, the resulting transient response may result in an undesirably large deviation between the actual yaw rate and the standard yaw rate, and this may give rise to an unacceptably large steering reaction.